1. Field of the Invention
This invention relates to a method of recognizing a radiation field on a stimulable phosphor sheet on which radiation image information has been recorded, in the process of reading out the recorded radiation image information by exposing the stimulable phosphor sheet to stimulating rays, causing the stimulable phosphor sheet to emit light, and photoelectrically detecting the emitted light.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as x-rays, .alpha.-rays, .beta.-rays, .tau.-rays, electron rays or ultraviolet rays, they store a part of the energy of the radiation. When the phosphor that has been exposed to the radiation is then exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in the applicant's Japanese Unexamined Patent Publication Nos., 55(1980)-12429 (U.S. Pat. No. 4,258,264) and 56(1981)-11395, it has been proposed to use such a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with the stimulable phosphor is exposed to a radiation passing through an object, such as the human body, to have a radiation image stored thereon, and the sheet is then scanned with stimulating rays such as a laser beam to cause it to emit light, which is photoelectrically detected and converted to an image signal that is used to reproduce a visible image of the radiation image on a recording medium such as a photographic light-sensitive material or on a display means such as a cathode ray tube (CRT).
This method using a stimulable phosphor sheet is advantageous compared with conventional radiographic methods using silver halide photographic material in that the image can be recorded over a very wide range of radiation exposure. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor is proportional to the amount of energy stored thereon over an extremely wide range, it is possible to obtain a radiation image regardless of the amount of exposure of the stimulable phosphor to the radiation, by reading out the emitted light with an appropriate gain and converting it to an electric image signal to reproduce a visible image on a recording medium such as a photographic light-sensitive material or a display device such as a CRT.
This system is also highly advantageous in that after the radiation image information stored on the stimulable phosphor sheet has been read out and converted to an electrical signal, the image signal can be appropriately processed to obtain a radiation image suitable for observation, particularly for diagnostic purposes, when it is used to reproduce a radiation image as a visible image on a photographic light-sensitive material or on a display device such as a CRT.
In such a radiation image system using a stimulable phosphor sheet, the read-out gain can be adjusted to an appropriate value when photoelectrically reading out the light emitted by the stimulable phosphor sheet upon stimulation thereof to produce a visible image. Therefore it is possible to obtain a reproduced radiation image that is not adversely affected by fluctuations in radiation dose caused by fluctuations in the tube voltage or MAS value of the radiation source, variation in the sensitivity of the stimulable phosphor sheet or the photodetector, a change in radiation dose according to the condition of the object, or a fluctuation in the radiation transmittance according to the object, and other such factors causing deviation in the level of the radiation energy stored on the stimulable phosphor sheet. Also, it is possible to obtain a desirable radiation image even when the radiation dose to the object is low.
However, in order to eliminate various effects caused by the fluctuation of radiographic exposure conditions and to obtain a radiation image having high image quality, it is necessary to ascertain the image input conditions of the radiation image stored on the stimulable phosphor sheet and the image input pattern as determined by the portion of the body, for example the chest or the abdomen, or the radiographic method used, such as plain image or contrasted image radiography, before reproducing the radiation image as a visible image for viewing, and then to adjust the read-out gain appropriately based on the detected input conditions and the image input pattern. The image input conditions and the image input pattern will hereinafter be referred to as the image input information when they are referred to generically. It is also necessary to determine the scale factor to optimize the resolution in accordance with the contrast of the image input pattern.
The image input information can be ascertained prior to the outputting of the visible image by use of the method disclosed in Japanese Unexamined Patent Publication No.58(1983)-67240 (U.S. Pat. No. 4,527,060). According to this method, a read-out operation for detecting the image input information of a radiation image stored on a stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is conducted beforehand by use of stimulating rays having a stimulation energy of a level that is lower than the level of the stimulation energy of stimulating rays used in a read-out operation for obtaining a visible image for observation (hereinafter referred to as the final read-out), and then, on the basis of the information obtained from the preliminary read-out, the final read-out gain is adjusted to an appropriate level, the scale factor is set, and the signal processing conditions are set.
Here, the description that the level of the stimulating rays used in the preliminary read-out is lower than the level of the stimulating rays used in the final read-out means that the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out is to be lower than the effective energy of the stimulating rays used in the final read-out. In order to make the level of the stimulating rays used in the preliminary read-out lower than the level of the stimulating rays used in the final read-out, the output of the stimulating ray source, for example, a laser beam source, may be decreased in the preliminary read-out, or the stimulating rays emitted by the stimulating ray source may be attenuated by an ND filter, an AOM or the like positioned on the optical path. Alternatively, a stimulating ray source for preliminary read-out may be provided independently of the stimulating ray source for the final read-out, and the output of the former may be made lower than the output of the latter. Or, the beam diameter of the stimulating rays may be increased, the scanning speed of the stimulating rays may be increased, or the transport speed of the stimulable phosphor sheet may be increased in the preliminary read-out.
Since with the aforesaid method the image input conditions and the image input pattern of a radiation image stored on the stimulable phosphor sheet can be ascertained in advance, even without using a read-out system having a particularly wide dynamic range it is possible to obtain a radiation image that is highly suitable for observation purposes by, on the basis of the recorded information, appropriately adjusting the read-out gain, setting the scale factor input information, and processing the detected electric image signal in accordance with the image input pattern.
A number of methods have been considered for ascertaining the information stored on the stimulable phosphor sheet on the basis of the preliminary read-out image signal obtained by the preliminary read-out. One such method involves preparing a histogram of the preliminary read-out signal. Specifically, as the stored information could be ascertained from signal maximum and minimum values and points of maximum frequency on the histogram, setting read-out conditions such as read-out gain and scale factor and image processing conditions on the basis of the histogram would enable the reproduction of radiation images having good diagnostic efficiency.
In the recording (photography) of radiation image information, with respect to the total recording region of the stimulable phosphor sheet, photography is usually performed with the field stopped-down to the radiation field in order to avoid the irradiation of diagnostically unnecessary portions, or to prevent contrast resolution being degraded by light scattering from diagnostically unnecessary portions intruding into diagnostically necessary portions.
However, the following problem arises when the above method is used to ascertain the information recorded on the stimulable phosphor sheet. With reference to FIG. 2, when radiation of a recorded image region of a stimulable phosphor sheet 103 is restricted to a field B and preliminary read-out is performed in respect of a region that is notably larger than the field B, for example the whole of the recording area, it gives rise to an incorrect grasp of what information is actually recorded within the field B. This means that in the above case a histogram will also include preliminary read-out signals from areas outside the field B. Therefore, in this case the histogram would not correctly reflect the information actually contained within the field B.
The applicant has proposed a number of methods for recognizing the above field B (for example, in Japanese Unexamined Patent Publication No. 61(1986)-39039) (U.S. Pat. No. 4,851,678). Use of such a method to recognize the radiation field and carry out preliminary read-out only in respect of the region of the said field enables the above problem to be resolved. However, in most of the conventional radiation field recognition methods the recognition of the field is carried out on the assumption that the field is trapezoidal in shape, and correct recognition of a field has been very difficult when the shape of the field is an irregular polygon.